1. Field of the Invention
The invention relates to an offset calibration method, and more particularly to an offset calibration for a radio frequency (RF) data path circuit.
2. Description of the Related Art
FIG. 1 shows a conventional RF data path circuit. As shown in FIG. 1, the RF data path circuit 1 comprises a variable-gain amplifier (VGA) 10, an equalizer 11, and an analog-to-digital converter (ADC) 12. The VGA 10 receives a signal RFIP, which is the sum of data signals A, B, C, and D from a pickup head, and a reference signal. The VGA 10 generates and transmits signals VGP and VGN to the equalizer 11. The equalizer 11 generates signals VADP and VADN according to the signals VGP and VGN. The ADC 12 generates a logic code LC by comparing the signals VADP and VADN. The equalizer 11 is an offset-sensitive circuit. When an offset voltage is present in the signals VGP and VGN, the equalizer 11 is not able to properly equalize the signals VGP and VGN. Moreover, even though there is no offset voltage in the signals VGP and VGN, the ADC 12 may generate an incorrect logic code when an offset voltage is originally present in the equalizer 11.
FIGS. 2a and 2b respectively show the waveforms of the signals VGP and VGN and the signals VADP and VADN if no offset voltages are present in the VGA 10 and the equalizer 11. When no offset voltage is present in the VGA 10 and the equalizer 11, the signals VGP, VGN, VADP, and VADN have ideal waveforms. Zero-cress points and duty cycles of the signals VGP and VGN are identical to those of the signals VADP and VADN. Therefore, a point P2a where the levels of the signals VGP and VGN are close to each other corresponds to a point P2b where the levels of the signals VADP and VADN are close to each other without point shift. The ADC 12 can correctly generate the logic code LC presenting “0” corresponding to zero-cross points P2a and P2b of signals VGP, VGN, VADP, VADN, thus avoiding errors in data slicing level.
FIGS. 3a and 3b show the waveforms of the signals VGP and VGN and the signals VADP and VADN when an offset voltage is present in the equalizer 11. When an offset voltage is present in the equalizer 11, subsequent to transmission of the signals VGP and VGN with ideal waveforms to the equalizer 11, the zero-cross points and the duty cycles of the signals VGP and VGN are different from those of the signals VADP and VADN. Therefore, a point P3a where the levels of the signals VGP and VGN are close to each other does not correspond to a point P3b where the levels of the signals VADP and VADN are close to each other. Correct determination of the data slicing level for the signals VADP and VADN is thus not possible. Moreover, due to the DC offset voltage, the signals VGP and VGN and the signals VADP and VADN are greater than expected levels, and saturation portions of these signals may cut out, resulting in poor data reading quality.
An RF data path circuit eliminating the offset voltage and enhancing the quality of data reading for an optical recording apparatuses is desirable.